Metal halides have a variety of uses, many of which require high purity. They are easily contaminated by water and oxygen and are generally handled in dry boxes.
One of the primary uses of metal halides is as a fill material in the arc tube of electric gas discharge lamps. Such lamps use a variety of fill materials including rare gases, metals, mercury amalgams and metal halides, and the proper operation of such lamps requires that these lamp fill materials contain less than 200 ppm oxygen, preferably less than 50 ppm, and less that 100 ppm hydrogen, preferably less than 20 ppm, and desirably less than 5 ppm.
The proper operation of such lamps also requires that the fill materials be inserted in precisely measured quantities during the fabrication of the lamps. Of particular concern is the vaporizable metal halide fill, generally in the form of pellets or particles. These metal halides determine the electrical and spectral characteristics of a lamp, and it is well known to select particular metal halides and their relative concentrations both to give light of a desired color and to impart desired electrical characteristics to the arc.
A typical metal halide arc tube contains a mixture of metal halides dosed as one or more spherical particles or a cylindrical pellet of precise composition and size.
Various methods are known for manufacturing both spherical uniform composition metal halide particles as shown in FIG. 1 and pellets (i.e., physically aggregated metal halides) as shown in FIG. 2.
If uniform composition and size is desired, the particles of FIG. 1 may be made by the apparatus schematically illustrated in FIG. 3, such apparatus and the manufacturing processes being of the type described, e.g., in the Anderson U.S. Pat. No. 3,676,534 dated July, 1972 and assigned to the assignee of the present invention, the content of which is hereby incorporated by reference.
In the process described in the Anderson patent, uniformly sized particles of metal halide mixtures are formed by forcing a homogeneous melt through an orifice of known diameter at a known velocity and acoustically or electro mechanically breaking the molten jet into controlled lengths.
Using this process of controlled jet break up, a mixture of DyI.sub.3, NdI.sub.3, and CsI with the CsI concentration greater than approximately 35 mole percent, forms particles which are quite weak, if particles are formed at all. These compositions typically have compressive breaking strengths, as measured by crushing a particle between two flat surfaces, of about 25 grams or less.
An alternative process described in the Anderson U.S. Pat. No. 4,201,739 dated May, 1980 and assigned to the assignee of the present invention, the content of which is hereby incorporated by reference. In that Anderson patent, particles are formed by the controlled wetting of an orifice which allows the dripping of molten metal halide spheres of a larger diameter.
Additionally, powders of a variety of metal halides may be aggregated by pressing into a cylindrical pellet or compacted tablet such as illustrated in FIG. 2 in a conventional mechanical device. (See, e.a., Friedrich U.S. Pat. No. 4,248,584). Finally, pellets for use in metal halide arc tubes have been produced by casting or by combining melting and pressing. (See e.g., Schaller U.S. Pat. No. 3,729,247).
It is known that the melting of metal halide mixtures can produce a homogeneous liquid, which if jetted, dropped, cast or pressed as described above, will produce Particles having a bulk composition that is the same as the original molten mixture. When a homogeneous liquid is rapidly frozen and ground into a powder it is in a suitable form for being pressed into a cylindrical or tablet shape.
For efficiency of manufacture, uniformity of dosage, and consistency of electrical and spectral characteristics of the lamps dosed, it is advantageous to combine several metal halides into a single particle or pellet. Each of these doses must have uniform composition to ensure consistent color and arc characteristics within the lamp and are dosed into the lamp arc tube by various mechanical schemes.
However, the metal halide particles and pellets, without regard to shape, (hereinafter referred to individually and collectively as "Particles") are frequently subjected to considerable mechanical abuse in the various manufacturing, handling, and dosing processes. To routinely withstand the rigors of manufacture, handling, shipping, and dosing the breaking strength of the metal halide particle needs to be greater than about 100 grams as measured by crushing a particle between two parallel surfaces, or through the use of a three point bend test. Certain metal halide mixtures do not form particles or pellets strong enough to withstand this mechanical abuse.
In addition, certain metal halide compositions can not readily be united into a single Particle. For example, a mixture containing iodides of dysprosium, neodymium, and cesium with a cesium content above approximately 35 mole percent could not be pressed into a solid pellet.
One reason for the fragility of metal halide mixtures is phase transformations. For example, a large volume change resulting from freezing of the particle may cause the particle to be in tension on its surface and under compression in its liquid interior. The result is often a particle with a large residual stress, which stress may lead to cracks or cleavage.
Other reasons for the fragility of the particle include the formation of weak and brittle intermediate phases, excessive numbers of cracks or voids, or both, and growth in preferred orientations that are weak.
Breakage on impact may be a significant problem in the formation of particles which are formed by a process in which the particles fall through a cooling tower and impact either the collecting vessel or previously collected particles. Depending on the need for spherical particles, significant waste may thus result in the formation and in the subsequent shipping and handling of such particles.
Without regard to the shape of the Particle being dosed, the implementation of the dosing of lamps with the desired metal halides in the desired quantity to obtain a desired color has heretofore suffered from the impression resulting from Particle breakage. Whether the fill is dosed by count or by volume, breaking of the fragile Particles during manufacture, shipment, handling and dosing may result in lamps with electrical and spectral characteristics other than those desired, and variation from lamp to lamp.
In addition to the variations which result from overdosing and underdosing of the lamps, breakage of the Particles may clog the dosing apparatus, disrupting the manufacturing process and wasting an expensive component of the lamps.
Because of the fragility of certain metal halide compositions, there is a great need for a strengthening agent that will not deleteriously affect the intended use of the Particle, e.a., in a lamp the agent must not significantly vary any of the arc characteristics, chemically react with the lamp electrodes, if present, or the walls of the arc tube.
It is thus highly desirable to strengthen the fragile Particles without negatively impacting the electrical and spectral characteristics of the lamp or the various arc tube components such as electrodes.
Attempts to strengthen metal halide particles without the addition of a strengthening agent have been partially successful. Annealing metal halide particles has produced a slight increase in the strength of certain particles, but the success of this procedure has been limited to relatively few materials. By controlling the freezing rate of metal halide particles, a few fragile compositions have been manufactured with improved strengths that allow them to be mechanically dosed. However, neither annealing nor control of freezing rate is universally applicable to metal halides.
It is accordingly an object of the present invention to provide a novel strengthening agent for metal halide Particles, a novel process for strengthening metal halide Particles, and novel compositions of Particles.
Another object of the present invention is to provide a novel composition for, and method of making, vaporizable metal halides useful in gas discharge lamps.
Yet another object of the present invention is to provide an strengthening agent for a vaporizable lamp fill which increases the breakage resistance of the material without significantly affecting the electrical and spectral characteristics of the lamp.
A further object of the present invention to provide a novel breakage resistant composition for metal halide Particles having particular utility as lamp fill material for metal halide lamps.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.